From personal experience, I've had balls strike, kill, then leave it alone for awhile, only to come back later to finish it off. How long did you leave it in with her? If you feed her in the evenings/night, I would have just left it alone till morning.

Then again, I keep thinking...maybe she changed her mind?

I wouldn't worry. She doesn't sound sick, she might have been waiting, or might have realized one was all she wanted. If you froze the rat immediately after she killed it, you can offer again next week, or in a few days.